Stainless steel manufacturing process and equipment



g- 15, 1967 c. w. M COY 3,336,132

STAINLESS STEEL MANUFACTURING PROCESS AND EQUIPMENT Filed March 9, 1964 Chromium Containing Slag 7 .Sl'nfered Iron Ore Raw Iron Ore High Chromium Ore L imeslone Gravel Cake Slag

Ha Mefa/ Hot Air Bldg 9 /maul Oxygen CLIFFORD W. MCCOY Alloy Additions Slag E Q ,5 Disposal Vacuum t /3 INVENTOR.

y l 9,44% Alta/nay United States Patent 3,336,132 STAINLESS STEEL MANUFACTURING PROCESS AND EQUIPMENT Clilford W. McCoy, Oakdale, Pa., assignor to Crucible Steel Company of America, Pittsburgh, Pa., a corporation of New Jersey Filed -Mar. 9, 1964, Ser. No. 350,297 11 Claims. (CI. 75-49) This invention pertains to new and improved processes for making stainless steels and, more particularly, to processes for the economical production of straight chromium and chromium-nickel stainless steels upon a large scale, continuous or semi-continuous basis.

The so-called stainless steels are relative newcomers on the metallurgical scene, having been developed at a comparatively late stage of development of the metallurgical industry. Thus, these steels have their genesis in the early years of the twentieth century and have been of commercial significance only since about 1916.

The stainless steels owe their generic name to their ability to effectively resist the attack of corrosive and oxidizing environments. This resistance to corrosive and oxidative attack is a direct result of the presence in these ir-on base alloys of relatively large quantities of the element chromium, and the degree of resistance to such attack is largely a function of the quantity of chromium present. About 11 or 12 weight percent chromium is required in order for these alloys to exhibit elTective stainless properties.

However, other properties of the stainless steels, such as room and elevated temperature strength, hardness, ductility, and general mechanical workability, are of great importance and, consequently, in the relatively short time since their commercialization, extensive research and development have resulted in introduction and use of a great variety of stainless steels which have, accordingly, assumed a role of vital importance in the technological aspect of modern civilization. Thus, iron base alloys containing as little as 9 or weight percent of chromium find application for certain purposes, particularly those wherein stainlessness is subordinate in importance to other properties, and the basic iron-chromium alloy has therefore been modified in a host of ways, by addition thereto of from one to several further alloying components in order to further enhance one or more properties of especial importance for particular applications.

The stainless steels are generally characterized as either (1) martensitic, (2) ferritic or (3) austenitic. Whereas the straight chromium steels are martensitic or ferritic in structure, others, by virtue of one or more alloying additions, possess an austenitic structure. Commonly produced and used austenitic stainless steel are those containing, in addition to about 14 to 24 weight percent chromium, relatively large amounts, e.g., 3 to 15 weight percent, of the element nickel.

The several different varieties of stainless steels may be, and often are, modified by the incorporation therein of other alloying elements, as manganese, silicon, nitrogen, sulfur, carbon, molybdenum, zirconium, titanium, columbium, tantalum, copper, vanadium or aluminum whereby one or more property characteristics of the basic steel composition is altered. In all cases, the stainless steels contain relatively large amounts of the element chromium which has a great chemical affinity for both carbon and oxygen. Certain others of the useful alloying additions to stainless steels, as silicon, titanium, aluminum, vanadium and columbium, also are very readily oxidizable, and are more reactive in this regard than the base alloy element, iron. Consequently, since their appearance, the bulk of the commercial quantities of the stainless steels have been produced by an electric furnace process which has been the sole commercially practical method of obtaining the low carbon contents almost universally required in these alloys. Due to inherent cost and production rate limitations of prior art processes, the production of stainless steels has always been more of a custom type of operation than, for example, the manufacture of plain carbon steel. Moreover, although many refinements have been added, over the years, to the original processes of stainless steel manufacture, current commercial processes retain, in their essential aspects, the character of processes developed twenty to thirty years ago.

Only a tiny fraction of the total stainless steel made today is produced by other than an electric furnace air melting procedure, e.g., vacuum are or vacuum induction practice.

The earliest, conventional electric furnaceprocess for stainless steel manufacture, the so-called dilution process comprised the melting, in an electric furnace, of carbon steel scrap followed by the addition of an oxidizing agent such as mill roll scale or iron ore or, in some cases chromium ore, to remove carbon by oxidation thereof to carbon monoxide. In this manner, under such an oxidizing slag, the carbon was first reduced to a required low level, whereupon the slag was then changed to a reducing slag such as lime and fiuorspar to which a reducing material such as ferrosilicon was added to reduce the iron oxide content in the slag. To this decarburized iron melt, the required chromium was then added, always under conditions to preclude addition of carbon to the melt. In order to keep the carbon low, it was necessary to add chromium in the form of low carbon material such as low carbon ferrochromium. This was a very expensive procedure. Therefore, improvements were made whereby higher carbon-containing, lower cost chromium source materials could be utilized. For example, processes were developed whereby the chromium was added to the molten iron bath in the form of either high carbon ferrochromium or stainless steel scrap. Such additions, of course, raised the carbon level of the molten bath to a considerable extent. Thereafter, the chromium-containing bath was oxidized to remove carbon, and, simultaneously, large portions of the chromium were oxidized and were incorporated in the slag. Thereafter, the chromium was reduced from the slag, e.g., by use of addition of reducing agents such as ferrosilicon. Such processes generally were carried out at relatively high temperatures, e.g., about 3250 F. in order to take advantage of the inverted oxidation susceptibility of carbon and chromium at such temperatures. However, such processes still require lengthy periods of holding the hot metal, with consequent uneconomical production rates and decreased refractory life. Furthermore, the slag products encountered are voluminous, also tending to reduce production rate and creating disposal problems.

Further refinements include the use of oxygen injection to oxidize the carbon. However, as in the case of the mill scale or iron ore processes, the oxygen process also results in excessive chromium oxidation and the necessity of a further, costly step to recover portions of the oxidized chromium from the overying slag.

Current electric furnace steelmaking practices for the production of stainless steel are inherently costly because they involve the melting of relatively large proportions of cold scrap or other chromium-containing solid materials such as ferrochromium. Electric power is, of course, an expensive source of energy. As indicated, current electric furnace stainless steel manufacturing practices are also inherently quite time consuming and therefore production rates are commensurately low. In addition to the cost of power, electrode cost and the cost of auxiliary electric power supplying control equipment and the high refractory cost involved, all contribute to the expense of this process and the consequent high cost of electric arc melted products. The development of processes enabling the use of stainless steel scrap was a significant contribution to the art, but even these processes are inherently costly inasmuch as the scrap raw material, variable in supply and price, must be used as a cold charge in combination with other even more expensive raw materials. Moreover, it is necessary to carefully select, segregate, store and handle scrap for stainless steel production by electric furnace melting to insure proper alloy chemistry and to meet the requirements of electric arc furnace operations with respect to scrap size, furnace volume and temperature limitations and associated refractory life.

Until relatively recently, all grades of stainless steel, from the straight 12 percent chromium steels to the most highly alloyed stainless steels, were considered as truly specialty steels, limited to applications wherein their superior properties of oxidation and corrosion resistance, hardenability, strength and/or ductility were absolutely essential. However, as above-noted, progressive research and development has resulted in enhanced productivity and improved economics of the electric arc melting process and in improved product quality. The adoption and widespread use of oxygen in electric arc furnace steelmaking is exemplary of such recent innovations. The field of application of stainless steels has thereby been tremendously increased with consequent demands for greater quantities of stainless steels and for a greater variety of stainless steels having new and improved properties and of lower cost.

Accordingly, it is an object of the present invention to provide new and improved processes for the manufacture of stainless steels.

It is a further object of the invention to provide new processes for making chromium and chromium-nickel stainless steels in a more economical manner than heretofore possible.

It is a more specific object of the invention to provide stainless steelmaking processes whereby the disadvantages of prior art electric furnace practices are avoided.

It is a further object of the invention to provide processes for producing stainless steels, in a continuous or a semi-continuous manner more directly than heretofore from the ultimate raw materials.

It is another object to provide a process for making stainless steel by utilizing lesser quantities of scrap or high cost ferrochromium or, alternatively, none at all.

It is a still further object of the present invention to provide an economical method for the large scale production of stainless steels having very low carbon contents.

It is a still further object of this invention to provide methods and means for economically producing stainless steels upon a magnitude and yield basis heretofore unknown to the art.

It is a still further object to provide a method of producing high-chromium pig iron directly from chromiumcontaining ore, or alternatively, from a combination of ore and scrap and/or ferrochromium.

It is yet another object of the invention to provide processes for producing low carbon-containing stainless steels from blast furnace effluent metal containing all or substantially all of the finally desired chromium or chromium and nickel contents.

It is yet another object of the invention to provide improved methods of producing substantially finished steel mill products from high-chromium blast furnace pig iron.

Still another object of the invention constitutes the provision of an improved method for making chromiumnickel stainless steels by oxygen vessel conversion of chromium-nickel blast furnace pig iron.

Still another object of the invention is the provision of economical methods for the production, at a rapid rate, and in large quantities, of hot metal containing stainless steelmaking quantities of chromium, with or without nickel, which metal is useful for direct conversion to low carbon, low silicon stainless steels, without the use of an electric arc furnace.

A more specific object is the provision of methods and apparatus for partially decarburizing high carbon iron base materials containing chromium, and with or without nickel, by limited oxygen blowing, followed by vacuum decarburization of the partially blown steel, to a final low carbon stainless steel composition.

In accordance with the foregoing objects, a preferred embodiment of the present inventive process comprises production, in a blast furnace, of an efiiuent metal containing stainless steelmaking quantities of chromium, with or without substantial quantities of nickel, together with substantial. quantities of carbon and silicon, blowing the effluent metal with oxygen at a time and temperature determined to oxidize all or most of the silicon and only a portion of the carbon content of the metal and to substantially saturate the metal with oxygen, and then treating the metal under vacuum whereby the carbon content is reduced to a very low level, all without substantial loss of chromium by oxidation.

A preferred embodiment of the inventive means comprises, in combination, means, as a blast furnace, to produce liquid, high chromium pig iron, with or without nickel, and containing substantial quantities of .carbon and silicon, means, as an oxygen converter, to burn a part of the carbon and/or the silicon from the pig iron, and vacuum means to complete the carbon removal to stainless steelmaking levels, while retaining in the iron all or substantially all of the original chromium content of the pig iron.

The foregoing and other objects of the invention will be more readily understood by references to the following detailed description, together with the single drawing which comprises a schematic flow diagram illustrative of the preferred method and means of the invention, and of the materials fiow resulting from the practice thereof, from the blast furnace, to the oxygen converter and then to the vacuum treatment operation.

Although the production of chromium-containing alloys in a blast furnace is known to the art, such prior art practices have been restricted to the production of ferrochromium materials intended for subsequent use in usual stainless steelmaking procedures, such as those hereinabove described. Thus, such prior art blast furnace products contained relatively large quantities of chromium, e.g. 30 to 70 weight percent chromium or even more. The procedural difliculties, such as the high furnace temperatures necessary to maintain slag fluidity and to sustain the chemical reactions, and associated extreme Wear on the furnace refractories and other parts, encountered in the production of such very high chromium materials, has had the result that such processes have gained no commercial significance.

However, the production of a blast furnace efiluent metal of a chromium content, with or without nlckel, Cast composition wejght substantially lower than those above described, and yet mQ Cast Dally ercent 1 Period No. Production, high enough for conferring the desired properties on Tons stainless steels made therefrom, is a practical and advan- 5 Cr 51 O S P tageous procedure. Thus, it is possible to produce, directly and economically in the blast furnace, a high carbomhlgh 7th day 4388 silicon, high chromium alloy for subsequent oxygen con- 1 version to stainless steel without the necessity for an g-g electric furnace melting or treating step. Moreover, such hot metal can be produced from low cost materials comprising iron ore, chromium ore, flux and reducing agent, without the need for using any high cost chromium source 8th day materials, as stainless steel scrap or ferrochromium.

The production of such chromium-containing pig iron in a blast furnace is illustrated schematically in the drawing, wherein the numeral 10 represents the blast furnace, and is exemplified by the data of Table I hereinbelow 4 wherein the operation of the blast furnace is divided into 9th day 4400 7 daily campaign periods. 4400 l l l g 4401 4402 4403 4403 4404 2. r 4405 2. TABLE I 10th day 4405 2.81 4407 2.53 Cast Composition, Weight 4408 2.91 Campaign Cast Daily Perceut 4409 2. 48 Period No. Production, 4410 3.07 Tons 4411 3.29

Cr Si 0 s P 11th day 4412 15.37 3.14 0.049 15.11 2.05 0. 051 Break-in 0.35 15.04 2.47 0. 051 period 2.15 14.53 3.25 0. 045 (1 day). 9.55 14.80 3.00 0.048 13.00 14.87 3.24 0.049 14.35

12th day 4418 14.52 3.98 0.050 15.02 2.95 0. 045 1st day 14.90 13.73 2.95 0.047 14.25 14.27 2.78 0.042 13.48 0.048 15.12 2.85 0. 054 14.40 0.049 15.40 3.09 0.050 14.08 0. 054 14.13 0,057 13th day 15.47 3.17 0.047 13.88 3.08 0. 055 2nd day"... 4360 8 0.066 17.86 14.58 0.051 14.17 3.38 0.071 15.10 0.049 15.35 3.51 0. 045 15.04 0,041 15.55 3.31 0. 051 14.53 3.10 0.048 2.93 0. 055 3rd (13. 15.17 3.09 0.049 14th da 2. 55 0. 048 y 14.30 3.17 0.055 y 14. 89 3.05 ,04 3.55 0. 045 15.07 2.77 0,051 13.52 2.40 ,049 3.08 0.045 13.50 2. 55 0,055 3.10 5. 0. 049 4th day 14. 81 3.11 0. 054 2.35 5. 0.039 14.75 2.95 0.052 3.30 4. 0. 052 14.57 2.27 0.055 14.80 2.45 0. 045 Shut-down' 3.17 4.53 0.008 0.073 14.38 2.14 0.054 period 3.14 4.30 0.005 0.079 14.97 2.51 0, 048 1 day). 3.35 0. 005 0. 089 2.15 5.50 0.005 0.092 5th day 15.14 2.64 0.057 2.14 4.40 0.007 0.089

1 Each figure is an average of two determinations of final cast analysis, 14:35 263 0:047v except where both determinations are given. $13? 3:51, 8: The initial blast furnace burden used to produce the fi-gg 5% 8-8;? cast compositions of Table I is given in the following 14:85 2:34 0051' Table TABLE II Weight, Composition, Weight Percent Burden Component 1 Lbs. Per

' Charge Fe Cr 8 Mn P 3104 A1204 MgO CaO Chromium Ore 0. 003 1. 71 15. 25 10. 88 0. 30 Taconite Sinter. 0.029 5. 50 0. 50 Limestone-.- 0. 002 0. 25 2. 45 52. 5 Gravel 0. 0&2 82. 00

1 Coke analyzed: 0.013 P, 3.07 SiOz, 2.46 A 0.13 MgO and 0.50 CaO. Ash=7.06%.

With the above-tabulated burden, the total iron input per charge was 6,850 pounds, and the total input chromium content was 1,530 pounds. Therefore, the charged chromium/iron ratio was 1,530/6,850=0.224. Taking, for example, the casts produced on the twelfth day of the campaign, as representatives of the contemplated process, and assuming a carbon content of 5.15% for cast number 4418, it is seen that the average Cr/ Fe ratio in cast numbers 43544359 (215 tons total weight) is 0.188. Although the Cr/Fe ratio in the cast metal was, as above shown, reasonably close to that in the furnace burden, such discrepancy as was observed is due principally to loss of chromium fines in the blast furnace flue dust. By attention to proper control of the particle size of the input chromium ore, this loss factor can be materially reduced. It is seen, therefore, that the Cr/Fe ratio in the blast furnace hot metal is commensulate to and regulable by control of the Cr/Fe ratio of the furnace burden input. This is of considerable importance, for, in contrast to the aforementioned prior art processes of making high chromium ferrochromium in a blast furnace, it is required, in the practice of this invention, that the furnace effluent metal comprise a relatively much lower, closely controlled percentage of chromium. Thus, the invention contemplates the production, in a blast furnace, of an eifiuent hot metal containing from about 9 to about 21 weight percent chromium, together with at least about 3.5% carbon and at least about 2% silicon, for reasons set out hereinbelow. Accordingly, for the purpose of the invention, the furnace burden is closely controlled, in respect of the Cr/ Fe ratio, to produce effiuent metal containing chromium within the above range. This requirement, together with limitations inherent in blast furnace operation, necessitates use of a furnace burden comprising chromium ore containing not less than about 20% by weight Cr O preferably not less than about 40% Cr O Alternatively, of course, the desired Cr/Fe ratio of the furnace burden may be established by use of stainless steel scrap and/ or high carbon ferrochromium as a portion of the chromium-containing charge material. Generally, however, the use of high chromium ore proves to be more economical.

In producing the Table I casts, the furnace was operated in accordance with the following dependent casts during the period of use of a higher chromium ore burden averaging about 16% chromium.

On the last day of the Table I campaign, the chromium ore burden was replaced by a normal basic iron burden, and return to plain carbon pig iron production is reflected by the decreasing chromium contents, from cast number 4434 to the end of the campaign.

The furnace burden may comprise a nickel-bearing material to produce chromium-nickel effiuent metals. Thus, I have tested the use of metal turnings in amounts up to 500 pounds per ton of hot metal as a furnace burden constituent in production of basic iron. From those tests, it is evident that stainless steel turnings, such as 18 percent Cr-8 percent Ni can be employed to raise the nickel content of the Cr/Fe hot metal as contemplated herein. Larger amounts of nickel, e.g. up to 800 pounds/ton, or even more, are feasible in such instances.

In accordance with the invention, high chromium, or chromium-nickel pig irons, such as those exemplified in Table 1, containing, preferably, as aforesaid, about 9 to about 21 weight percent chromium, up to about 8 percent nickel, about 3.5 to 6 percent carbon and about 2 to 5 percent silicon, are delivered to an oxygen converter wherein the metal is subjected to an oxygen blowing step, with the two-fold objective of (1) eliminating all or most of the silicon content (the oxidation of which element is highly exothermic and hence serves as a convenient thermal energy source in the oxygen blow), and preferably, a large part of the carbon (also a fuel source), and (2) substantially saturating the molten metal with oxygen-the significance of the latter requirement being made apparent hereinbelow. The oxygen blow is conveniently and preferably accomplished by means of available top-blowing processes and apparatus such as the socalled L-D equipment (as schematically shown in the appended drawing wherein the converter is denoted generally by the numeral 11), or the K-aldo process, the Rotor process, the Ajax process or any equivalent of any of these. Alternatively, the oxygen blow may be done by bottom or side blowing means and methods, as a Bessemer converter. A top-blowing oxygen-treating process is highly preferred, not only for the usual technical reasons advanced in favor of such processes over other, older processes, but for the additional or supplementary variables. reason that the temperatures requ1red 1n the oxygen 'blow TABLE III Operating Variables Campaign Period, Furnace Burden 1 Days Avg. Wind Avg. Coke Avg. Blast Rate, Rate, Avg. Blast Pressure, s.c.f.m. Lbs/Ton Temp., F. p.s.i.

Metal Cast 15 Table II 28, 500 3, 059 1,175 11.1 6-9- Table II, except: 28, 000 3,235 1, 220 10. 4

5,2001bs./charge Or ore. 10-12 .do 3,235 1,200 11.2 13 Table II, except: 30,000 1, 180 11.1

6,000 lbs/charge Cr ore. 14. Return to plain carbon pig commenced.

1 Exclusive of coke. 2 Increased from 28,000 to 33,000 (avg. 32,000).

The absence of significant change in cast metal chromium content after the initial increase in chromium ore content of the furnace burden is believed due to the concomitant increase in coke rate plus an observed increase in moisture content of the chromium ore due to adverse weather conditions.

However, the second increase of chromium ore in the burden was, as shown in Table I, accompanied by a significant increase in chromium content of the cast metal,

7 are quite high in order to avoid undue oxidation of the chromium content during oxidation of the silicon and carbon. Thus, it is known that, although chromium is relatively more oxidation-susceptible at lower temperatures, at sufficiently high temperatures, this oxidationsusceptibility relationship undergoes a reversal, whereby carbon oxidizes preferentially to chromium. The relationship between carbon content, chromium content and i.e., up to 17.8% chromium (cast number 4425), the temperature during the oxidation period of stainless steel melting is well-known, having been established in connection with prior art electric furnace practices using oxygen blowing for decarburization. Thus Hilty et a]. have reported the relationship as:

Percent Or -13,800

Percent C T +8'76 (Equation 2) By use of Equation 1 or 2, it becomes possible to determine the equilibrium contents of carbon and chromium, as a function of temperature, as well as nickel content (in the ease of Cr-Ni steels) in the molten metal in the oxygen converter, as contemplated by'this invention. It is thereby determined that, in the oxygen conversion of the blast furnace elfiuent metals herein contemplated, containing the aforesaid limited quantities of chromium, with or without nickel, the converter temperature must be upwardly of about 3200 F., preferably in the range of 3200-3600 F. Thus, for the production of stainless steels, by a direct conversion, from the blast furnace efiiuent metal, to a final steel composition containing about 20 weight percent chromium, and as little as 0.10 percent carbon, Equation 1 shows that a converter temperature upwardly of 3400" F. is required. Such temperatures are extremely deleterious as regards useful life of the converter refractories. In order to operate a converter, even a top blown converter, at a temperature of, say 3400-3600 F., operation must be limited to extremely short time periods. Even then, refractory life is so affected as to make commercial operation under such conditions subject to a definite economic disadvantage. The dilficulty in this regard, it must be noted, is most pronounced when attempts are made to produce stainless steels ofhighest chromium and lowest carbon contents. Unfortunately for the technical considerations involved, many stainless steels require such low carbon values, e.g., well below 0.10 weight percent.

Therefore, although blast furnace manufacture of high chromium pig iron affords an extremely economical and productive source of chromium metal-in contrast to the usual, more expensive procedures by means of which chromium is generally placed in form and condition for production of stainless steels, for example, by common prior art electric arc melting practices which utilize either ferrochromium, ferrochcromium silicon or stainless steel scrap of variable chromium cntent-the use of such materials in a direct conversion, duplex process, although useful under certain conditions, e.g., where best compositional control is not required, or where process economics are not controlling, has definite and significant limitations. Thus, illustrative of such a duplex process, 6,000 pounds of a high chromium pig iron, produced in a blast furnace, was remelted in a cupola, the molten metal analyzing 14.77% chromium, 5.05% carbon and 2.05% silicon, plus residual quantities of sulfur, phosphorus and manganese. After melting, this melt was then cast into a 10,000 pound capacity, top-blown oxygen converter, lined with high alumina brick (about 95% A1 0 5%Si0 at a starting temperature of 2400 F. Twenty-five pounds of Fe O were added as a starter to 1 D, C. Hilty, AIME Trans., 1949, vol. 185. p. 91. D. C. Hi1ty G. W. Healy, and W. C. Crafts, AIME Trans, 1953, vol. 197, p. 649.

D. C. Hilty. H. P. Russbach, and W. C. Crafts, J. Iron & Steel Inst, 1955, vol. 180, p. 116.

Simkovioh and C. W. McCoy: AIME Trans., 1961, vol. 221, p. 416.

aid ignition, and oxygen was then introduced through a vertically-positioned lance, having the tip thereof located about twelve inches above the molten surface. Immediately upon ignition, pounds of lime were added, followed by a further 160 pounds during the blowing period. A total of 6,750 standard cubic feet of oxygen were delivered to the converter in 16.5 minutes, for an average oxygen delivery rate of 26,130 standard cubic feet per hour or 8,710 s.c.f.h. per net ton of charge.

At the end of the blowing period, at which time the metal temperature was in excess of 3200 F., as indicated by melting of the measuring thermocouple, a sample of the metal taken at termination of the blowing period showed the following analysis: chromium 11.58%, carbon 0.091% and silicon 0.005%. Thereafter the vessel was partially deslagged and 4 pounds of aluminum and 14 pounds of titanium added, followed by an addition of 9.5 pounds of electrolytic manganese and 11.0 pounds of silicon. After a 15-minute cooling period, a SOD-pound pig was poured followed by a 500-pound hot-topped ingot measuring 10 x 10 x 18 inches. The remainder of the metal was poured into 500-pound pigs. Subsequent analysis of the ingot was as follows: carbon .11%, chromium 12.03%, manganese 22%, silicon .16% and titanium .05 The slight increase in carbon content is attributed to carbon pickup from the mold stool and the slight chromium increase is attributable to chromium reduction from remaining slag by titanium. Slag analysis showed an iron-to-chromium ratio of 0.35 in the final vessel slag. Using this ratio, together with initial and final metal composition, an oxidation loss of 13.6% and a chromium recovery, during conversion, of 67.7% was determined.

The economic disadvantage of an approximately 30% loss of chromium is apparent. Of course, a significant portion of the chromium values oxidized into the slag during oxygen-blowing to the carbon-chromium equilibrium point can be recovered, for example, by addition to the slag, after blowing, of a chromium-reducing ma terial, such as silicon, e.g., in the form of a ferrosilicon or an Fe-Cr-Si alloy. However, such chromium recovery steps are both costly and time consuming and, moreover, are incapable of reducing all of the oxidized chromium within practical economic time limitations In addition to the aforesaid disadvantages of a direct ox-ygen conversion of blast furnace chromium-iron pig to stainless steel, the required very high temperatures are, as above noted, extremely deleterious as regards life of the converter refractories. Indeed, presently available refractories simply cannot be subjected, for economically significant periods of time, to temperatures on the order of3400-3600 F. Even if means are used to quickly ignite the bath and rapidly raise its temperature to the required high value, thereby to shorten the high-temperature exposure of the refractories, the very rapidity of such operations makes it difficult to accurately control the final composition of the oxygen-blown product, consequently making it necessary to adjust the final composition by added operations, as by further treatment in an electric furnace.

Application of Equation 1 shows that, at the lower temperature of 3200 F., 0.10% carbon is productive of an equilibrium chromium content of 9.3%, while, at a temperature of 3300 F., in a nickel-free bath, a chromium content of about 14.1% can be obtained with an equilibrium carbon content of 0.100% in the final steel composition, and 9.2% chromium can be obtained with 0.065% carbon, and, at a converter temperature of 3400 F., a carbon level of 0.10% in the final stainless steel composition is productive of an equilibrium chromium content of 20.9%. Steels of still greater chromium content and/ or steels with commensurate chromium content, but of lower carbon content, require, of course, still higher temperatures for oxygen blowing to final, equilibrium composition.

The final stainless steel products contemplated by the present invention, include those containing chromium in a range of from about 9 to about 21%, or even greater, as up to about 25%, but preferably about 12 to about 18%, together with carbon contents below about 0.15% to 0.20%, preferably below 0.10%, although, in certain instances, the carbon content of the final steels may be much higher, e.g., about 0.60 to 1.20%, as in the case of the modified AISI 440 type cutlery steels, as types 440A, 440-B, 440-BM, 440-F and 440-F-Se. The aforementioned limitations of elevated temperature refractory life, and consequent economic considerations, restrict the current practical maximum converter operating temperature when the hot metal is blown directly to a desired final composition, to about 3375 F. Such a temperature is seen to be insufiiciently high to be useful in the production, especially, of the higher chromium, low carbon stainless steels for which temperatures of from 34003600 F. are needed.

It has now been found possible to avoid the aforesaid difiiculties, and still economically produce highly desirable stainless steels having extremely low carbon values and, if desired, relatively high chromium contents. This is done, pursuant to the inventive principles hereof, by subjecting high chromium, or chromium-nickel pig irons containing substantial amounts of carbon and silicon, to only a partial or limited oxygen blow, thereby removing all or most of the silicon content thereof, as well as only a portion of the carbon content, and thereafter completing the reduction of carbon to the desired low level by a further step comprising treatment of the partially blown, oxygen-saturated, metal under a substantial vacuum.

Illustratively, a 16,000-pound charge of a high carbon, high silicon, chromium-iron blast furnace product was melted in an electric arc furnace (in commercial practice of the invention, molten blast furnace metal would preferably be used directly), then the molten metal transferred to an approximately -ton capacity basic oxygen converter of the top-blown type. The metal was blown at a constant rate of 80,000 s.c.f.h. after an initial addition of 1100 pounds of lime. A further addition of 700 pounds of lime was made after the first sampling of the metal after commencement of oxygen blowing. The process factors and chemistries involved are given in the following Table IV.

1 Including interruptions for sampling and temperature measurements, Total blowing there was 12 minutes.

The total weight of the product after completion of the limited blowing was 13,750 pounds, representing a product yield of 86%. The chromium recovery was 93.1%. Thereafter, the steel was poured into a ladle with deslagging, and alloyed by ladle additions of 40 pounds of aluminum, 90 pounds of titanium, 120 pounds of 0% ferrosilicon and 60 pounds of electrolytic manganese. The metal was then poured into a 24-inch by 27-inch ingot having a composition: 0.35% carbon, 13.04% chromium, 0.56% silicon, 0.51% manganese, 0.24% titanium, balance substantially iron.

A further charge, of size and character as above, was similarly melted and 1700 pounds of lime added for slag making and preheating purposes, and the metal-slag mixture then top-blown as above-described. The initial composition was 4.28% carbon, 1.19% silicon, 11.68% chromium, balance substantially iron with usual steelmaking impurities. The metal was oxygen-blown, at a constant, uninterrupted rate of 80,000 s.c.f.h., for about 13 minutes, for a total oxygen input of 17,000 s.c.f. The blown metal was thus tapped into a ladle, to which 40 pounds of aluminum, pounds of titanium, pounds of 50% ferrosilicon, and 60 pounds of electrolytic manganese, were made, and the product then teemed into a. 24 inch by 27 inch ingot which was analyzed and found to have a composition as follows: 0.16% carbon, 054% silicon, 11.50% chromium, 0.25% titanium, 0.40% manganese, balance substantially iron.

A still further 1600 pound charge was melted, having a composition: 4.41% carbon, 0.98% silicon, 13.29% chromium, balance substantially iron. Eleven hundred pounds of lime were added to the molten metal which was then oxygen blown, again at a constant, uninterrupted rate of 80,000 s.c.f.h., for about 12 minutes (oxygen input of 16,500 s.c.f.). The blowing rate was then reduced to 50,000 s.c.f.h. for three minutes, giving an additional oxygen input of 2000 s.c.f., for a total of 18,500 s.c.f. After tapping into a ladle, additions of 0 pounds of aluminum, pounds of titanium, 100 pounds of 50% ferrosilicon, and 60 pounds of electrolytic manganese were made, and the product teemed into a 24-inch by 27-inch ingot which was found to have the composition: 0.12% carbon, 0.37% silicon, 13.22% chromium, 0.55% titanium, 0.45% manganese, balance substantially iron.

A similar procedure, wherein an-initial charge consisting of 4.56% carbon, 1.07% silicon, 14.05% chromium, balance substantially iron, was oxygen-blown at a rate of 30,000-60000 s.c.f.h. (total oxygen input of 24,000 s.c.f.), gave a blown product analyzing 1.30% carbon, 0.01% silicon, 11.20% chromium, balance substantially iron. Consequently, the higher oxygen blowing rate is preferred for more efficient carbon removal and chromium recovery.

A procedure such as that exemplified hereinabove has been found to have certain highly desirable results. Thus, termination of the oxygen blowing step short of complete reduction of carbon to the necessary, final stainless steel level, permits operation of the oxygen blowing step at temperatures substantially lower than would otherwise be required, thereby effecting a tremendous savings of converter refractory life and cost. For example, in the production of a 20 weight percent chromium stainless steel, partial blowing of the high carbon, high silicon converter input metal to an intermediate carbon value of 0.20 weight percent, permits operation of the converter at a theoretical maximum temperature of only 3200 F.a temperature well within the range productive of reasonable life expectancy of available converter refractories. The economic advantage is self-evident.

Additionally, by the practice of this invention, with the concomitant permissive use of much lower converter temperature than heretofore considered possible, it is entirely feasible and possible to produce stainless steels having highest chromium contents, e.g., up to about 25 weight percent. Thus, whereas a duplex process requires a converter temperature on the order of about 3 00 F., in order to obtain a final carbon content of 0.07 percent, such steels are readily producible, by a partial oxygenblowing, whereby carbon is reduced to an intermediate value, for example, about 0.15 percent, using a converter maximum temperature of only about 3350 F. The carbon level of such partially refined steels is then further reduced to the desired level by vacuum treatment as more fully described hereinbelow.

Moreover, further advantage is realized in the form of increased chromium yield. Despite the reversed oxidation susceptibility of carbon and chromium at elevated converter operating temperatures, oxygen blowing inevitably is productive of quite appreciable chromium oxidation. This fact, of course, represents an economic loss reflected in a high cost of the final production of converter-produced stainless steels. Use of the relatively much lowei' converter temperatures permissible in accordance with a limited blowing step results in a much smaller loss of chromium.

Moreover, such a limited oxygen blow constitutes. a practical, economic utilization of a blast furnace effluent metal containing, as above described, large amounts of silicon as well as carbon. Thus, in the course of an oxygen blowing step, the exothermic rections taking place are represented by the following, standard state equatrons:

C+ /2O CO+26.84 kcal./gm. mol. Si+O SiO +20L4 kcal./gm. mol. (Equation 4) Fe+ /20 FeO+64.3 kcal./gm. mol. (Equation 5) Cr+%O /2Cr O -|-121.9 kcaL/gm. mol. (Equation 6) Thus, by operating at relatively low converter temperatures, and by thereby minimizing the highly exothermic (Equation 3) oxidation of chromium, the silicon oxidation reaction tions 3 and 4, inherent in the blast furnace production of products containing economically highly advantageous .quantities of chromium and, if desired, nickel, contribute most significantly to the economic feasibility of the inventive process. Therefore, the blast furnace effluent metal, for use in the process of this invention, is required to contain at least about 1%, preferably about 2 or 3% to about 5% or more, by weight, of silicon, in addition to the aforesaid quantities of carbon. Fortunately, such quantities of these elements are readily obtainable when the blast furnace is operated on high chromium or high chromium-nickel burdens, as aforesaid. Thus, it has been found that, in order to effectively reduce the high chromium ores as exemplified hereinabove, blast furnace temperatures are required which are appreciably higher than those required in the same or similar installations for the production of plain carbon pig iron. Commensurately, these higher operating temperatures result in generally higher percentages of silicon so that silicon contents greater than 2% are readily obtainable. Furthermore, the combination of higher temperature, together with high chromium content, is productive of higher percentages of carbon in the resultant hot metal. The initial or starting compositions set out in Table IV and the succeeding examples show silicon contents lower than 2%, due to oxidation loss of silicon during electric furnace remelting of the blast furnace pig. In commercial practice, of course, the molten blast furnace metal would be used directly.

The temperature of the molten bath in the converter can conveniently becontrolled, within the aforementioned desired range, by the addition of controlled quantities of scrap metal to the molten bath. Conveniently, such scrap additions may be of the same or substantially the same composition as that of the bath itself or of the final desired product. Alternatively, alterations to the bath chemistry may be made at this point in the process by addition of scrap having a composition other than that of the molten bath. The temperature of the. molten bath in the oxygen converter can, of course, be controlled in other ways, such as the addition of ferroalloys or ores or changes in oxygen blowing rate. It will be seen, therefore, that the converter charge metal may usefully have a composition other than that which, after oxygen blowing, would be directly productive of the desired final stainless steel composition. Thus, converter charge metal relatively deficient in alloying elements, such as chromium, may be produced and the desired enrichment achieved by scrap or other alloying additions as described hereinabove.

The partially refined metal, resulting from a limited, low temperature oxygen blow, as above described, still contains substantial quantities of carbon-far above the carbon levels required in most stainless steels. However, such partially refined materials do have the highly reactive silicon content substantially removed. This is of considerable importance in view of the preferential oxidation of silicon over carbon and the consequent inability to effectively further lower the carbon content of melts containing appreciable quantities of silicon, as will be shown hereinafter. Moreover, oxidation of silicon is productive of voluminous slag formation. Absence of both such possibilities is of importance inthe further stages of the inventive process as will become apparent.

Attempts have been made heretofore to effect the refinement of stainless steel melts, e.g., those produced in accordance with usual electric furnace practices, by completion of the carbon reduction process under vacuum, to promote the reaction of Equation 3 above.

Thus, it is known that reduction of the partial pressure of gaseous oxides of carbon over a molten steel bath promotes the reaction of Equation 3. However, in order for the aforesaid reaction to proceed to the extent that carbon is removed from the steel bath to the necessary low levels required in stainless steels, it is necessary that oxygen for the reaction be readily available. Availability of oxygen to combine with carbon has heretofore been the insurmountable obstacle encountered in such attempts at vacuum purification. It is believed, but it is to be understood, of course, that the invention is not limited by any particular chemical, physical or kinetic theory, that the heretofore encountered difficulties are due to a number of causes. If the molten bath contains substantial quantities of elements which are relatively more readily oxidizable than carbon under the particular process conditions, these elements will combine with oxygen and lower its content in the bath such that not as much is available for the Equation 3 reaction. Unfortunately, such elements are commonly present in stainless steel melts, either as intentional alloying additions, or as impurities. Silicon is exemplary of such elements. The invention, however, provides, as hereinabove described, a bath for final refinement (and amenable to further alloying with such highly reactive elements) wherein such reactive elements are absent or substantially so. Thus, such elements are effectively, cheaply and efiiciently eliminated by the limited oxygen blowing step, which is carried to the point where such reactive elements are reduced to such low values that they constitute no material hindrance to the reaction of Equation 3. This fact is essential to the proper operation of the inventive process.

Secondly, even in the absence of elements which impede the functioning of the Equation 3 reaction, oxygen cannot and will not enter into that reaction unless it is in a proper form, and is present under proper circumstances favorable to the reaction. For example, addition of particulate oxygen-containing solid materials, as mill roll scale, iron ore, nickel oxides, manganese oxides, etc., to molten stainless steel baths under subatmospheric pressure, have been found to be ineffective to reduce the carbon level to the required low levels. Although the reasons for the failure of such approaches to this problem are not definitely known, it is believed that the oxygen content of such additives is prevented from combining with carbon due to the rapid formation of relatively impervious blocking layers of comparatively inert oxygen compounds about the particle surfaces. Such inhibiting reactions in chromium-containing steel melts are believed to involve the formation of chromium-oxygen or ironchromium-oxygen compounds, with or without other elements, as manganese, for example, chromium oxides (Cr O .Cr O chromite (FeO.Cr O or, if manganese is present, the corresponding manganese compounds or manganese-chromium compounds, e.g., MnO.Cr O The presence in the bath of appreciable quantities of silicon possibly lead to the formation of complex compounds, as rhodonite (MnO.SiO or fayalite (FeO.SiO as well as silicon dioxide (SiO all of which possibly act, and in a manner more profoundly than that which would be stoichiometrically indicated, to alter availability of oxygen added in chemically combined, solid, particulate form.

However, the molten bath, after being subjected to the limited oxygen blow pursuant to the present invention, is substantially saturated with dissolved oxygen which, in the absence of more reactive elements removed by the blow,.is readily available for combination with the remaining carbon when the bath is subjected to the contemplated vacuum treatment.

Illustrative of the latter step of the invention, there were prepared several heats of a chromium-containing steel saturated with oxygen, at a temperature of 3000 F., representing the product of a typical, limited oxygen blowing step as herein contemplated. Containers of these steels, each holding thirty pounds of molten steel, at the stated 3000 F. temperature, were placed in a vacuum chamber and the chamber evacuated. In each instance, the metal commenced boiling at a pressure of about 300 mm. Hg. When boiling began, the vacuum pumps were throttled to hold the pressure substantially constant until boiling subsided, whereupon pressure was again reduced until boiling again occurred. This procedure was repeated until a final pressure of 0.1 mm. Hg was reached. Boiling substantially subsided, in most instances, upon the attainment of a pressure of about mm. Hg and, in all cases, at pressures below about 2 mm. Hg. Each of the steel samples was chemically analyzed for carbon, chromium and silicon contents: (1) initially (after melting), (2) intermediately (after oxygen saturation), and (3) finally (after vacuum treatment), The results are given in the following Table V, the compositional balance, in each case, being, of course, iron.

TABLE V Composition, Weight Percent Sample No.

C Cr Si 0.07 14. 88 N11 0.075 13. 32 Nil 0. 024 13.03 Nil 0.053 15.01 Nil 0.055 13. 43 Nil 0. 019 13. 50 Nil 0.038 14.76 Nil 0. 034 13. 29 Nil 0. 014 13. 38 Nil 0. 066 14. 77 N11 0. 066 13. 73 Nil 0.007 13. 37 Nil 0. 11 15.04 Nil 0.106 14. 05 Nil 0.033 13.86 Nil Nil 0. 22 15. 06 Nil 0.11 15. 10 N11 0. 14 14.36 Nil 0.13 14. 36 Nil 0.021 14. 46 Nil The vacuum decarburization reaction proceeds violently and rapidly to eflFectively reduce carbon to a desired low level heretofore impossible on a large commercial scale in such an efficient and economical manner. From the results of the Table V tests, it will be observed that the vacuum treatment as described was efiective to lower the intermediate carbon level of the oxygen saturated steels to extremely low levels-well within the limits required for all or practically all stainless steel compositions, and, correlatively, at only very little sacrifice in chromium content.

Additional, similar experimental heats were made to each of which was added (immediately after the oxygen blow) a quantity of silicon, in order to determine the TABLE VI Sample Composition, Silicon Wt. Percent Added, Wt.

Percent Final C/Initial 0, Percent Sample No.

Although, the data of Table VI seem to indicate an adverse efiect of increasing silicon content of the molten metal on decarburization during vacuum treatment, the trend is not consistent. However, comparison of Samples H and L, as well as Samples K and M, the individual samples of each pair having about the same initial carbon level, does indicate the significance of the silicon effect on final/initial carbon ratio. Moreover, it was noticed, during the Table VI tests, that the degassing time decreased. Accordingly, it was believed that the slag remaining on the metal bath and on the furnace side walls may have provided a source of oxygen for decarburization to the observed minimum values, despite the presence of increasingly large quantities of silicon. Accordingly, still further tests were performed, similar to those above described, but wherein the slag layer was removed from the metal bath prior to the addition of silicon, and the silicon-containing, slag-free baths then transferred to the vacuum treatment chamber.

Such steel compositions, together with the test results, are given in Table VII.

TABLE VII Sample Composition, Silicon Wt. Percent Final Sample No. Added, Wt. C/Initial 0,

Percent Percent Si 0 Cr 17 silicon content of the blast furnace effluent metal be substantially completely removed in the oxygen blowing step of the inventive process.

The solubility of oxygen in percent chromium stainless steels is approximately 0.06 percent at usual steelpouring temperatures, e.g., about 2800' F. This quantity of oxygen, theoretically, would be productive of a carbon decrease of about 0.04 percent in accordance with Equation 3. Yet further tests were performed, wherein steels of varying chromium level were made (-five pound heats), the steels blown with oxygen to saturation and then analyzed for, inter alia, oxygen. The results are given in Table VIII.

It is noted that, in each case, the oxygen content of the blown steels was much higher than would be expected from known oxygen-solubility data. The reason for this discrepancy is not known, but it is believed that it may be due to the presence in the blown steel compositions of metal oxides, possibly including those of chromium, as would be indicated by the reduction in chromium content of the steels after blowing. Possibly a fraction of oxides so produced remain dispersed, in extremely finely divided form in the molten metal.

Nickel-containing austenitic stainless steels, as well as the straight chromium steels, are similarly amenable to the contemplated vacuum treatment step of the invention. Thus, the invention contemplates the production, in accordance with the above-described three-step or triplex steelmaking process, of austenitic, nickel-bearing steels, such as the several steels of the A.I.S.I. Type 200 and 300 series, as well as the martensitic or ferritic chromium steels of the A.I.S.I. Type 400 series. Other stainless steels, such as the straight 12 percent chromium grades are also contemplated, as well as other chromium-bearing steels for special applications.

In the commercial practice of the invention, the converter 11, after partial deslagging, as shown in the drawing, is emptied of its metallic contents, the latter being poured into a vessel suitable for subsequent vacuum treatment of its contents, e.g., a ladle 12, as also illustrated in the drawing. The latter is then removed to the vacuum treating zone, into operative association with the vacuum equipment.

Any suitable means may be utilized for performing the vacuum treatment of the invention. Various types of equipment and processes 'which are currently available may be used, or the same may be modified in any desirable or necessary fashion. Other expedieuts obvious to those skilled in the art are, of course, within the purview of the invention. Those processes and means which are best adapted to create a large surface area for the' carbon-oxygen reaction and for degasification of the molten steel, are preferred. Illustrative of a preferred method and means for carrying out the vacuum treatment is the so-called D-H device and process wherein the liquid metal to be treated is contained in a ladle into which depends a refractory ceramic tube through which the metal is forced upwardly into an overlying vacuum chamber. Such equipment is schematically illustrated in the drawing, the numeral 13 denoting generally the vac uum treating equipment, the depending tube being identified by the numeral 14. Suchequipment and process is well adapted to maximize surface area exposure for the above-stated purposes. This effect, in the above-described or other vacuum apparatus and processes, can be still further increased, if required or desired, by injecting into the molten metal undergoing vacuum treatment an inert, low atomic weight gas, such as hydrogen.

Other vacuum treatment devices and processes may be used in the performance of the invention, for example, a vacuum stream degassing step, or merely exposure of a container of the partially refined steel to subatmospheric pressure in an evacuated chamber.

It is contemplated that, after performance of the vacuum treatment, as above described, or in conjunction therewith after substantial completion of the carbonoxygen reaction and degasification and deoxidation, alloying additions of relatively reactive elements, as titanium, aluminum, calcium, magnesium, manganese, zirconium, vanadium, rare earth metals or silicon may be made to the refined metal bath. Loss of such elements, as by unwanted oxidation, by adding them to the contemplated products at this stageof their manufacture, is substantially eliminated. It is also contemplated that substantial further additions of chromium or nickel can be made to the melt at this point in the process, as well as additions of relatively less reactive alloying elements, as copper, molybdenum, columbium, tantalum, etc. The latter elements may, of course, also be added to the molten metal at an earlier stage of the process, as after the limited oxygen blow. Non-metals, as sulfur, boron, nitrogen, etc., may also be added, preferably after the vacuum treatment has been completed. Addition of the more highly reactive alloying additions when necessitated in the final steels for any particular property requirement, is definitely preferred after vacuum treatment for the added reason that addition thereof prior to completion of the required carbon-oxygen reaction would inhibit that reaction by preferential oxidation of such more oxidation-susceptible elements, thereby preventing achievement of the desired low carbon levels. Thus, resiliconizing is preferably accomplished in this manner, as well as the incorporation of alloying or deoxidizing elements as titanium, columbium, or aluminum, all of which elements are commonly or occasionally required in one or another of the contemplated stainless steels. The so-called DH equipment is particularly preferred for the additional reason of the relative ease of making such alloying additions, as by means of an evacuable hopper 15 atop the vacuum chamber.

The addition of relatively large percentages of difiicultly soluble alloying elements to the molten metal bath at one or another stage of the inventive process, as above described, may require a supplemental heat input to the metal bath to attain complete solution. This may be conveniently accomplished by removing the molten metal to an electric arc furnace, as a separate step of the process, or, alternatively, additional heat may be supplied in conjunction with one of the aforesaid process steps, e.g., by providing holding means for the metal in the vacuum chamber and heating the metal, while so held, for example, by radiation, induction, etc.

The tremendous advantages of the invention are best realized by performance in accordance with the steps hereinabove described. However, the broad principles of the invention also encompass alternatives to the aboveexemplified steps.

Thus, the converter input metal, although preferably constituting a blast furnace product, as described, may be produced by an electric furnace or even an open hearth or cupola process, for example, by melting, in such furnaces, an iron base material containing the required quantities of chromium, with or without nickel, and by adding the necessary quantities of carbon and silicon for performance of the subsequent oxygen blowing step. The

latter two elements may be added to such heats in the form of, e.g., carbon electrode pieces, or high carbon or h gh silicon ferroalloys. The invention also contemplates, in its broader aspect, production of a converter input metal by a submerged arc electric furnace smelting procedure, or by a combination thereof with melting furnace". as aforesaid. Such procedures, of course, are, generally, more difiicult, time consuming and costly than the use of blast furnace metal, although definite advantages are realizable with the use of a converter input metal produced by cupola-melted metal or by the product of a submerged are electric furnace process. However, blast furnace metal, as has been shown, is directly productive of a converter input metal having the required quantities of carbon, from about 3.5 to about 6 weight percent, and silicon, about 2 to 5 weight percent, which are useful as fuels to supply the necessary thermal energy for the oxygen blowing. Such blast furnace products, depending upon the nature of the ores used in their production, contain more or less manganese. This element also is oxidized during the oxygen blowing step and, if desired in appreciable amounts in the final steel compositions, can conveniently be re-added after completion of refinement as above-described.

Similarly, as above set forth, a forceful oxygen blow in a suitable converter is preferred in the performance of the invention, but alternatives are possible within the broadest contemplation of the invention principles. For example, a partially refined heat of a desired stainless steel composition may be made by an electric furnace, open hearth, cupola, or other practice as mentioned above. Preparatory to final vacuum refinement to reduce the carbon content of such partially refined material to a desired low value, and after the carbon and silicon contents of the partially refined material are reduced to relatively low but not final values, e.g., 0.1 to 0.3 percent carbon and less than 0.1 percent silicon, the molten metal may be substantially saturated with oxygen, as by a comparatively slow bubbling or diffusion of oxygen into the the bathin contradistinction to the use of a forceful jet of oxygen to itself remove substantial quantities of carbon and/or silicon. Such an oxygen-saturated product can then be vacuum treated as above described.

It is important, however, as above shown, that the input metal to the vacuum treatment operation be substantially free of silicon, since the presence of substantial quantities of that element will cause reaction with the oxygen present and thereby prevent occurrence or completion of the desired carbon-oxygen reaction. Moreover, oxidation of quantities of silicon is productive of voluminous slag formation which not only impedes effective vacuum treatment, but is also quite deleterious to the refractory linings used in vacuum treatment equipment.

The invention also is inclusive, in its broader aspects, of processes wherein the second and third steps, as above described, i.e., oxygen saturation and vacuum treatment of the molten metal, are performed simultaneously. For example, a partially-refined melt, that is, one wherein silicon is substantially absent and carbon is at an intermediate level as above-described, is placed within an evacuated chamber and therein subjected to a relatively slow injection, as by bubbling or diffusion, of oxygen whereby the carbon combines with the injected oxygen and the resulting gaseous carbon oxides removed under vacuum. Depending upon the nature of the apparatus used, the oxygen may be mixed with an inert stirring gas, as argon, nitrogen, etc., to promote the carbon-oxygen reaction kinetics. Alternatively, superheated steam may be utilized which, upon dissociation, provides oxygen for carbon removal and hydrogen for bath agitation.

The foregoing description and specific embodiments are merely illustrative of the principles of the invention, and it is to be understood that other embodiments, modifications and additions may be made by those skilled in the art without departing from the spirit and scope of the Invention.

What is claimed is:

1. A method of producing chromium-bearing stainless steel having a carbon content less than about 0.10 percent, comprising:

(a) producing an initial alloy comprising, by weight percent, about:

Percent chromium 9 to 25 nickel Up to 8 carbon 3.5 to 6 silicon 1 to 5 iron, balance, except for impurities.

(b) blowing the alloy with oxygen to reduce the carbon content thereof to a value intermediate that of the initial alloy and that required in the final steel composition, and to reduce the silicon content of the blown alloy to a value sufficiently low that silicon will not prevent substantially complete stoichiometric reaction between the oxygen dissolved in the alloy upon termination of oxygen blowing and the excess remaining carbon over that required in the final steel composition, and

(c) subjecting the blown alloy to subatmospheric pressure to complete the reduction of the carbon level thereof to the desired final value.

2. A method in accordance with claim 1 wherein the pressure is not greater than about 2 mm. Hg.

3. A method in accordance with claim 1 wherein the initial alloy is produced in a blast furnace.

4. A method in accordance with claim 3 wherein the minimum silicon content of the initial alloy is about 2 percent.

5. A method in accordance with claim 1 wherein the oxygen blowing is accomplished by means of a topblowing process.

6. A method in accordance with claim 5 wherein the oxygen top-blowing step is carried out at a maximum temperature below about 3400 F.

7. A method in accordance with claim 5 wherein the oxygen top-blowing step is carried out at a maximum temperature below about 3200 F.

8. A method of making a stainless steel containing, in weight percent, about 9 to about 21 percent chromium, up to about 8 percent nickel, and carbon less than about 0.10 percent, comprising:

(a) producing ina blast furnace, a molten iron-base alloy consisting essentially, by weight percent, of

about:

Percent chromium 12 to 25 nickel Up to 8 carbon 3.5 to 6 silicon 2 to 5 iron, balance, except for impurities.

(b) oxygen top-blowing the molten alloy at a maximum temperature below about 3400 F. for a time sufiicient to reduce the silicon content to less than about 0.10 percent and the carbon content to less than about 0.20 percent but substantially above that required in the final steel composition, and

(c) removing the molten, oxygen-blown alloy to a vacuum-treating zone and there subjecting the molten alloy to a subatmospheric pressure less than about 30 mm. Hg and for a time sufficient to reduce the carbon content of the alloy to the value desired in the final steel composition.

9. A method in accordance with claim 8 wherein the final pressure in the vacuum-treating zone is not greater than about 2 mm. Hg.

10. A method in accordance with claim 8 wherein, after substantial completion of the vacuum treatment, the molten steel is further alloyed with elements more oxidizable than carbon at the temperature of the molten steel.

11. A method in accordance with claim 8 wherein the References Cited UNITED STATES PATENTS Rohn 7549 X Vogt 7549 X Royster 75-130.5 Harders 7549 22 Armbruster et a1 7549 Josso 75-49 Spolders et a1 7549 X Jandros 75--l30.5 Finkl 7549 Krivsky 751305 DAVID L. RECK, Primary Examiner. HYLAND BIZOT, Examiner. Bridges 7560 H; w. TARRING, Assislant Examiner. 

1. A METHOD OF PRODUCING CHROMIUM-BEARING STAINLESS STEEL HAVING A CARBON CONTENT LESS THAN ABOUT 0.10 PERCENT, COMPRISING: (A) PRODUCING AN INITIAL ALLOY COMPRISING, BY WEIGHT PERCENT, ABOUT: 